With the development of information technology, communication traffic through which large-volume data is transmitted and received at high speed using light is beginning to be rapidly prepared. In particular, the volume of data has been significantly increased not only in conventional communication networks (telecommunications), but also in data communications such as Storage Area Network (SAN) and Ethernet (registered trademark) (LAN), and high-speed communications using light have been actively progressed. For example, the communication standard of 100-gigabit Ethernet for transmitting 100-gigabit data is beginning to be designed in the next-generation optical LAN.
Further, the throughput of a recent high-end router used in a backbone system reaches as high as 1 Tbsp, and further expansion of capacity is expected in the future. Along with this, optical interconnections show great promise to efficiently process large-volume data in data transmission between these transmission devices (a few meters to hundreds of meters) or in short-range data transmission in the device (a few centimeters to tens of centimeters). While large-capacity systems using light have been progressed as described above, low-cost technology becomes more important to provide data transmission using these devices at low costs.
With such a background, improvement of high-speed performance and simple and high-density mounting are important issues for a semiconductor optical element transmitting and receiving signals. The reason for this is as follows. With the increased speeds and capacities of systems, an optical element itself will face a physical limit in high speed in the near future.
Therefore, it is necessary to transmit data using signals of plural channels instead of conventional one channel. For example, a configuration of using 40 channels, in each of which data is transmitted and received at 25 Gbps, has been envisioned for the above-described high-end router. Accordingly, a high-speed semiconductor optical element excellent in high density and simple mounting will become one of key devices in large-capacity systems in the future.
The semiconductor laser element that is an optical source for signal transmission is classified into three types depending on combinations of cavity directions (vertical resonance and horizontal resonance) and faces from which laser light is emitted (end-face emitting and surface emitting). The first type is a horizontal-cavity end-face-emitting laser element, the second type is a vertical-cavity surface-emitting laser element, and the third type is a horizontal-cavity surface-emitting laser element.
In the horizontal-cavity end-face-emitting laser of the first type, an optical waveguide is formed in the direction parallel with a surface of a substrate, and laser light is emitted from an end face obtained by dividing the substrate by cleavage. In the laser structure, the length of the cavity can be increased to as long as hundreds of micrometers, and a high output of tens of megawatts can be obtained even at high temperatures. However, it is necessary to install an optical member for receiving laser light in the surface of the mounting substrate while being adjacent to the laser element, and thus the laser is not suitable for multi-channel high-density mounting and downsizing of the entire module.
Next, the vertical-cavity surface-emitting laser of the second type is a laser having a structure in which the cavity is formed in the direction orthogonal to a semiconductor substrate. Therefore, a light receiving member can be installed at an upper surface of the element, and the laser is advantageous in high density in the surface of the mounting substrate. In the case of this structure, however, the length of the cavity is extremely short because it is determined on the basis of the thickness of a crystal growth film, and it is essentially difficult to obtain high optical output.
The horizontal-cavity surface-emitting laser of the third type has a laser structure that combines excellent features of the above-described two lasers. In the structure, the cavity is formed in the direction parallel with a surface of a substrate, and a reflecting mirror inclined at 45° is integrally formed to emit laser light from the top or rear surface of the substrate.
The present invention relates to the horizontal-cavity surface-emitting laser of the third type. As an example of such a conventional horizontal-cavity surface-emitting laser, Patent Literature 1 discloses a horizontal-cavity surface-emitting laser including an active region with 10 to 100 μm, a distribution Bragg reflector, and an inclined mirror. Further, as a second well-known example, Non-patent Literature 1 reports room-temperature continuous oscillation characteristics of a horizontal-cavity surface-emitting laser including an optical waveguide having an InGaAsP active layer formed on an InP substrate, a reflecting mirror formed at an end portion of the optical waveguide while being inclined at 45°, and a circular lens formed at a position facing the 45°-reflecting mirror on the rear surface of the InP substrate.
Furthermore, as a horizontal-cavity surface-emitting laser having a structure and a mounting mode different from those of the above-described lasers, Non-patent Literature 2 discloses a horizontal-cavity surface-emitting laser of the type in which both of p- and n-electrodes and a light emitting surface are provided at an upper surface of a substrate. In the laser, the surface of the substrate of the chip is bonded onto a mounting substrate using AuSn or Ag epoxy, and then p- and n-electrodes are connected to p- and n-polarities on the mounting substrate, respectively, using gold wires.